One of the things that I like about Christopher Hughes's A Complex Theory of a Simple God, a discussion of the doctrine of divine simplicity in Aquinas, is that he largely manages to approach the problem of what idem means in Aquinas in a respectable way. Rather than merely assuming he knows what it means, he tries to build up a body of textual evidence for his interpretation, and does a reasonable job of it. I think his interpretation is (probably) wrong, but it's a good attempt.
The basic problem Hughes is addressing is this. Aquinas distinguishes idem secundum rem from idem secundum rationem; these notions, different ways of being the same, are also not mutually exclusive. How do we understand all these different notions of being the same? Hughes argues that identitas secundum rem is classical identity. If this is so, however, we face some obvious problems, because Aquinas very clearly denies of identitas secundum rem several obvious properties of classical identity. For instance, if X is identical to Y, and Y is identical to Z, X is identical to Z. But Aquinas is vehement that this transitivity does not hold for identitas secundum rem. A puzzle.
The puzzle has a natural solution that Hughes rejects. I think this natural solution is (probably) the right one, although there are one or two issues that still remain unresolved. The solution is this: classical identity, identity in our sense, is identitas secundum rem et rationem, not identitas secundum rem. So I'd like to look briefly at this position, without getting into more than some very basic reasons for accepting it.
First, some background. Here, as elsewhere with medieval technical terms, it's usually a good idea to start with the least technical translation of the term, and then refine it in light of the various distinctions and qualifications made by the medievals themselves. Ultimately in medieval Latin, the best translation for identitas, at least at first approximation, is sameness. Aquinas and others aren't starting with our sense of 'identity' and adapting it to other uses; instead they are starting with a basic term (idem, same) and analyzing it for various purposes. Our sense of the word 'identity' is one of the historical scions of their analysis, not their starting point. And we find very clearly that medieval thinkers usually don't mean identity when they talk about identitas -- usually, in fact, they seem to mean sameness of kind. But not always, of course, and that's where it gets somewhat interesting.
The road from Thebes to Athens and the road from Athens to Thebes is the same road; yet in a sense the road from Thebes to Athens is obviously not the road from Athens to Thebes. How should we understand 'same' in this context? How should we understand the difference? According to Aquinas, the road from Thebes to Athens and the road from Athens to Thebes are the same road secundum rem (roughly: according to the thing), but differ secundum rationem (roughly: according to the notion). The road from Thebes to Athens doesn't have exactly the same properties as the road from Athens to Thebes -- indeed, necessarily they will sometimes be different. For instance, if the road from Thebes to Athens is uphill, the road from Athens to Thebes has to be downhill. Direction, by which we notionally distinguish the T-A road from the A-T road, does make a difference, and, indeed, a real difference. On the other hand, there aren't two roads here; they are the same road.
Of course, as Hughes notes, we can take "The road from Thebes to Athens is uphill" to mean "The road from Thebes to Athens is uphill-in-a-Thebes-to-Athens-direction"; he argues that, since one and the same road is uphill-in-a-Thebes-to-Athens-direction and downhill-in-a-Thebes-to-Athens-direction, this shows that the road from Thebes to Athens and the road from Athens to Thebes are indiscernible -- the road from Thebes to Athens has exactly the same properties as the road from Athens to Thebes. But I think this is too glib. Of course the T-A road is going to be the same road as the A-T road; but it doesn't follow from this that they are indiscernible, nor that the relation between the two is identity in our sense. Hughes seems to me to make a common mistake of assuming that because identity in our sense can be clearly and precisely characterized, it can be clearly and easily applied. But in fact it is not so. It is very difficult to know when two things, A and B, that are the same in some way, should be characterized by saying "A is identical to B". And there are lots of contexts in which it does seem to make a difference whether you are talking about the road from Thebes to Athens or the road from Athens to Thebes, which there shouldn't if identity were (unproblematically) applicable here. Moreover, they do appear to have different properties. It's not because the road is the road from Thebes to Athens that it is the road from Athens to Thebes; it would be entirely possible for there to be two one-way roads in each direction. So, while this road that is the road from Thebes to Athens is obviously identical to this road that is the road from Athens to Thebes, the notions 'the road from Athens to Thebes' and 'the road from Thebes to Athens', under which this single road is conceived, are very different. This means that, while we can clearly find an identity here in our sense of the term, we have to do it in a roundabout way. And this suggests that Aquinas is right to sense a distinction here. The road from Athens to Thebes and the road from Thebes to Athens are the same secundum rem but differ secundum rationem.
The only form of sameness recognized by Aquinas that acts like identity in our sense is identitas secundum rem et rationem. And this makes sense, because this is the only sort of sameness that unproblematically and obviously has the features we attribute to identity. Aquinas allows that this sort of sameness has transitivity and intersubstitutability, for instance, which can't be guaranteed of two things that are only the same secundum rem, or only the same secundum rationem.
Hughes does a valiant job arguing that we should take identitas secundum rem as identity; but the cost of this is a long string of very forced interpretations of Aquinas. His most significant set of arguments, however, are quite significant. Namely, he argues that some of Aquinas's arguments, which rely wholly on identitas secundum rem, only work if identitas secundum rem is classical identity. One of the arguments is this one:
And as the divine simplicity excludes the composition of subject and accident, it follows that whatever is attributed to God, is His essence Itself; and so, wisdom and power are the same in God, because they are both in the divine essence. (ST 1.40.1 ad 1)
As Hughes summarizes it, the argument would be: wisdom in God is the same as the divine essence; power in God is the same as the divine essence; therefore wisdom in God is the same as power in God. But this doesn't seem quite right. Hughes's interpretation leaves out part of the argument, namely, that "as divine simplicity excludes composition of subject and accident, it follows that whatever is attributed to God is His essence Itself." Since whatever is attributed to God considered as subject is the same as the divine subject (by the noncomposition of subject and accident), and wisdom and power are both such things (because they are in the divine essence) they are the same in God, namely, in virtue of each being the very same divine subject. This seems enough to make the argument go through; you don't have to treat the sameness of wisdom and power as identity in our sense. The divine subject is identical with itself, regardless of what is attributed to it. So the part of the argument Hughes leaves out in its summary helps to strengthen the conclusion that can be drawn.
Another argument Hughes considers:
But since relation, considered as really existing in God, is the divine essence Itself, and the essence is the same as person, as appears from what was said above (39, 1), relation must necessarily be the same as person. (ST 1.40.1 c.)
As Hughes summarizes this, a relation is the same as the divine essence, the divine essence is the same as a person; therefore a relation must be the same as a person. The problem with this summary is that it ignores the fact that this argument is a response to another position, namely, one in which the personal relations are externally affixed or 'assistant' to the divine essence and the divine persons. Aquinas's only concern in this argument is to insist that this is simply false. And his argument is adequate for this without dragging identity into the matter. If relation is the divine essence itself, and the essence is (in whatever way) the same as the person, necessarily, the relation must (in some way) be the same as the person, and thus not externally affixed or 'assistant'.
Most of the textual evidence Hughes adduces for his view is less promising than these, being more ambiguous. So, while I like the attempt, I don't think he's established what he thinks he has.